Some people have a really dirty mind.
You could always have clarified that you was just drumming your thigh and not masturbating, and I thought there was a decent difference in the sound produced..
Fyl

Why the hell would you ruin the chain of funny comments?^ And when I masturbate in the stall, I leave the door open so that there is no question as to whether I'm beating my dick, or drumming on my thighs.